...DISCUSSION... ...Wednesday/Day 4 to Friday/Day 6... The ECMWF, GFS, Canadian and UKMET models are in general agreement for Wednesday, moving an upper-level trough into the mid Mississippi Valley. Surface dewpoints ahead of the trough are forecast to be in the 60s F. As surface temperatures warm during the day, a pocket of moderate instability may develop across the mid Mississippi Valley by afternoon. This combined with 30 to 40 kt of deep-layer shear should be enough for an isolated severe threat from parts of Missouri into Illinois and western Kentucky where a 15 percent contour has been added. On Thursday and Friday, the medium-range models move the upper-level trough eastward into the Northeast and off the Atlantic Coast. The models suggest a front will be located from the north-central U.S. into the Ohio Valley. This would be the favored corridor for thunderstorm development each afternoon, along which an isolated severe threat can not be ruled out. Any severe threat would depend upon the timing of shortwave troughs and the magnitude of destabilization.

...Saturday/Day 7 to Sunday/Day 8... On Saturday, the models suggest that an upper-level trough will be located in the western U.S. with west to southwest mid-level flow over the central and northeastern states. The ECMWF shows a corridor of maximized low-level moisture from the lower Mississippi Valley into the upper Ohio Valley while the GFS has the moist sector further west. Severe thunderstorms will be possible Saturday afternoon along the northern edge of the moist sector where mid-level flow is forecast to be stronger. The models vary considerably on how far north the moist sector will be on Saturday. On Sunday, the models move the upper-level trough across the Rockies and maintain west to southwest mid-level flow from the Mississippi Valley eastward. The models suggest a front could be located from the southern Plains extending northeastward to the Ohio Valley. Severe thunderstorms would be possible along the boundary where the greatest instability develops Sunday afternoon. However, uncertainty is considerable at this range in the forecast period.

Latest GFS is showing potential for this time period to end up with a more significant event.

This looks intense, but nothing happens because everywhere east of the dryline has a strong cap.

As we saw with the April 3 event... the lack of convection in the Plains enhances the severe threat for the Midwest/OV/GL the next day because the EML is allowed to spread east without being interfered with by convection.

Sounding just south of Fort Wayne, IN.

Edit: SPC actually did mention this time period in their disco.

QUOTE

...Saturday/Day 7 to Sunday/Day 8... On Saturday, the models suggest that an upper-level trough will be located in the western U.S. with west to southwest mid-level flow over the central and northeastern states. The ECMWF shows a corridor of maximized low-level moisture from the lower Mississippi Valley into the upper Ohio Valley while the GFS has the moist sector further west. Severe thunderstorms will be possible Saturday afternoon along the northern edge of the moist sector where mid-level flow is forecast to be stronger. The models vary considerably on how far north the moist sector will be on Saturday. On Sunday, the models move the upper-level trough across the Rockies and maintain west to southwest mid-level flow from the Mississippi Valley eastward. The models suggest a front could be located from the southern Plains extending northeastward to the Ohio Valley. Severe thunderstorms would be possible along the boundary where the greatest instability develops Sunday afternoon. However, uncertainty is considerable at this range in the forecast period.

A cap is still forecast to stop convection in the Plains on the 11th and 12th, allowing an EML to spill into the OV.

Lots of negatives with this setup. The jet stream is largely front-parallel, which supports linear storm mode. The way the system is oriented is also a negative.. clearly a positive tilt. Should help keep the forcing limited to the front... at best. Speed shear is enough for organized storms... directional shear is okay in the lowest kilometer or so but unidirectional above that.

On the positive side... nice EML overlapped with upper 60/lower 70 dew points leads to the strongest instability this region has seen so far this year. With low LCL and "okay" directional shear, could be some tornadoes with the squall.

Enhanced risk at best IMO. All of the problems I listed wouldn't be a thing if the system wasn't so positively tilted.

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS EASTERN MISSOURI AND PARTS OF ILLINOIS...

...SUMMARY... Scattered thunderstorms associated with severe hail and isolated wind damage will be possible across parts of the mid Mississippi Valley on Wednesday.

...Mid Mississippi Valley... A shortwave trough is forecast to move across the Mississippi Valley on Wednesday. At the surface, a cold front is forecast to advance eastward across Iowa and Missouri with a corridor of maximized low-level moisture in place ahead of the front. Surface dewpoints may be able to reach the low to mid 60s F across parts of the mid Mississippi Valley by afternoon. If this occurs, then pockets of moderate instability could develop in areas that warm sufficiently. The increasing instability combined with enhanced low-level convergence along and ahead of the front should result in scattered thunderstorm development from west of the Chicago area southward to around St Louis. This convection is forecast to move eastward across the slight risk area during the late afternoon and early evening. The NAM appears to be aggressive with moisture and instability. Taking this into account would yield MLCAPE values in the 1500 to 2000 J/kg range across western Illinois by late Wednesday afternoon. This combined with 25 to 35 kt of 0-6 km shear and steep low-level lapse rates could be enough for multicells associated with isolated wind damage. Severe hail may also occur especially if moderate instability can be realized across the slight risk area.

GFS and NAM have some impressive parameters. Still skeptical with the low being so far away from the area and the cold front being so weak. Only real way you can tell where the front is is by the wind shift. Otherwise, nice southerly flow with westerlies aloft.

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF THE MIDDLE AND LOWER MO VALLEY...

...SUMMARY... Marginally severe thunderstorms will be possible on Tuesday from southeast South Dakota south-southeastward into the lower Missouri Valley.

...Synopsis... Shortwave trough currently moving through the northern Rockies/northern High Plains is expected to turn southeastward late tonight and move across the northern/central Plains and into the upper/mid MS Valley on Tuesday. Surface low attendant to this shortwave is expected to move across central SD into southern/central MN during the period while a secondary surface low moves across central KS into the mid MO Valley. Surface trough connecting these two features will act as an effective frontal boundary, moving southeastward across much of the central Plains. Temperatures differences on either side of this boundary will be minimal but convergence along the boundary may contribute to isolated to widely scattered thunderstorms across the central Plains. Thunderstorm development is also possible within the narrow warm sector across the MO Valley.

...Mid/Lower MO Valley... A narrow warm sector is expected to develop over the region between the approaching surface trough and the drier air farther east across the mid MS Valley. Modest, evapotranspiration-aided low-level moisture is forecast to be in place by the early afternoon. Given the origins of this low-level moisture, there is some question to its depth and whether or not model progs of its presence can be trusted. That being said, consensus amongst both the medium-range and ensemble guidance is for dewpoints in the upper 50s to low 60s. This low-level moisture, when combined with steep mid-level lapse rates and temperatures in the upper 70s/low 80s, supports at least moderate instability (i.e. MLCAPE around 1000 J/kg). Warm-air advection, encouraged by a low-level jet moving into the region as well as subtle forcing for ascent provided by the approaching shortwave trough, will provide the impetus for convective initiation during the afternoon and evening. The displacement between the better low-level moisture (south) and the stronger forcing for ascent (north) as well as differences in the guidance moisture quality results in some uncertainty regarding overall convective coverage across the region. As a result, the Marginal Risk area was expanded slightly.

Aforementioned dynamic low-level jet expected to move into the region during the afternoon, enhancing low-level flow but the flow above about 800 mb is quite weak, resulting in relatively weak overall bulk shear. This lack of stronger mid/upper-level flow is expected to mitigate the overall severe potential as any storms that do develop will likely struggle to maintain organization and transition to a more linear/cluster character quickly. Even so, thermodynamic profiles support the potential for hail with initial updrafts. Additionally, backed surface winds and adequate low-level flow also support the potential for brief tornado.

...Central SD/NE... A few strong storms are possible along the surface trough as it moves across the region during the afternoon and evening. However, weak vertical shear and modest moisture (i.e. dewpoints a few degrees less than areas farther east) should limit instability and storm organization, mitigating overall storm strength.

...sounding for tomorrow ...1800UTC...trying to figure out how to read these still...interesting though...near Virgil, SD...

Sounding shows a lot of mostly elevated instability (instability is when the air parcel (dotted line) is warmer than the environment temp (red line))

Low-level winds are weak but out of the north and back (turn clockwise) with height, which tells you cold air advection is ongoing

Winds throughout the profile are weak (<30 knots), so organized storms are possible but not really likely. With how much instability there is, I'd imagine hail would be the primary threat but not a very good hail threat. Also might be some strong winds due to how dry the profile is.

For the April 11-13 event, looks like GFS has changed the story. It was showing a frontal passage through the region from the west but now looks like we're dealing with a stationary front that sags a little bit south with storms running along the front.

Sounding shows a lot of mostly elevated instability (instability is when the air parcel (dotted line) is warmer than the environment temp (red line))

Low-level winds are weak but out of the north and back (turn clockwise) with height, which tells you cold air advection is ongoing

Winds throughout the profile are weak (<30 knots), so organized storms are possible but not really likely. With how much instability there is, I'd imagine hail would be the primary threat but not a very good hail threat. Also might be some strong winds due to how dry the profile is.

For the April 11-13 event, looks like GFS has changed the story. It was showing a frontal passage through the region from the west but now looks like we're dealing with a stationary front that sags a little bit south with storms running along the front.

Parameters are there for supercells... it's just the number of cells that pop that is the question.

Sounding about half way up the IL/IN border

It's interesting that the SPC has the outlook centered over IL but many of the hi res models such as the 3km NAM, WRF-ARW, NMMB, and WRF NSSL have much of the convection well east in IL into much of IN which is a result of quicker timing.

It's interesting that the SPC has the outlook centered over IL but many of the hi res models such as the 3km NAM, WRF-ARW, NMMB, and WRF NSSL have much of the convection well east in IL into much of IN which is a result of quicker timing.

Thought the same thing. 12z NAM/3k NAM are still more east. We'll see if SPC shifts the risk area east on the updated day 2 in the next 90 minutes.